IMM Report Number 33: Nanomedicine

Could Medical Nanorobots Be Carcinogenic?

By Robert A. Freitas Jr.
Research Scientist, Zyvex Corp.

Biocompatibility is an important property that must be carefully engineered into all medical nanorobots used in nanomedicine [1, 2]. One key aspect of biocompatibility is whether implanted nanoorgans, or in vivo medical nanorobots, might induce undesirable genetic changes as a side effect of their presence or activities inside the human body. Such undesirable changes might take many forms. For instance, mutagenicity [3] is the production of inheritable coding flaws in chromosomes that otherwise may retain much genetic functionality. (All carcinogens are mutagens but not vice versa – a mutation may be lethal to a cell, may prevent cellular replication, or may not affect metabolic or growth processes sufficiently to produce malignant behavior [4].) Genotoxicity is a more serious injury to the chromosomes of the cell, such that when the cell divides, fragments of chromosomes and micronuclei remain in the cytoplasm. Teratogenicity [5] is the ability of a foreign material (or a fetotoxic agent) to induce or increase the risk of developing abnormal structures in an embryo, or birth defects. Carcinogenicity is the ability to produce or increase the risk of developing cancer – materials may be directly carcinogenic or may potentiate other agents [4]. Tumorigenic materials tend to induce neoplastic transformations, especially malignant tumors.

Direct experimental exploration of the carcinogenicity of likely nanorobot building materials has barely begun, but information available to date appears guardedly optimistic. For example, diamond (DLC) coatings exhibit low mutagenicity toward human fibroblasts in vitro [6] and there are no reports of diamond carcinogenicity or tumorigenesis. Alumina (sapphire) produces no mutagenic or carcinogenic effects on cultured human osteoblasts [7] or when used as a blood-contacting material in a centrifugal blood pump [2]; while aluminum ion leached from sapphire at the highest plausible concentrations (~10-5 M; [2]) might inhibit eukaryotic transcription, experiments suggest that the mutagenicity, carcinogenicity, and teratogenicity of aluminum is low [8]. Teflon particles appear to be noncarcinogenic [2], even though tetrafluoroethylene (a monomer used in Teflon manufacture) is hepatocarcinogenic after long-term inhalation by mice [9]. There are no reports of carcinogenicity from pyrolytic carbon, graphite, or pure India ink in humans [2]. In rodents, the inhalation of carbon black particles can produce pulmonary neoplasms and lung carcinoma [2], and particle-elicited macrophages and neutrophils can exert a mutagenic effect on in vitro rat epithelial cells [10].

The possible carcinogenicity of fullerenes was suggested a decade ago [11] but even by 1998 the risk was no longer considered serious [12]. Pure C60 and C70 molecules do not intercalate into DNA (which might promote cancer) when mouse skin is exposed to them [11], although water-miscible fullerene carboxylic acid can cleave G-selective DNA chains [13]. No mutagenicity or genotoxicity of C60 as fullerol is observed in prokaryotic cells and only slight genotoxicity is seen in eukaryotic cells at the highest concentrations – even though C60 dissolved in polyvinylpyrrolidone was found to be mutagenic for several Salmonella strains due to singlet oxygen formation, and pure C60 is a known singlet oxygen generating agent, and singlet oxygen is known to be genotoxic [2]. Repeated epidermal administration of fullerenes for up to 24 weeks resulted in neither benign nor malignant tumor formation in mice, but promotion with a phorbol ester produced benign skin tumors [11]. Some C60 derivatives have actually shown promise as anti-cancer or anti-tumor agents [2]. Carcinogenicity studies of rolled graphene sheets such as carbon nanotubes remain to be done.

There are four kinds of carcinogenesis [4] which may be relevant in medical nanorobotics:

(1) Chemical Carcinogenesis. Chemical carcinogenicity is actually a somewhat uncommon property of materials. An exhaustive literature search on 6000 of the most likely chemical candidates found only 1000 (17%) identified as possible carcinogens [4]. The classic study by Innes et al. [14] found that fewer than 10% of 120 pesticides and toxic industrial chemicals tested were carcinogenic, and even this study was criticized as being too pessimistic because testing toxic potential carcinogens at high dosages may artificially accentuate their activity by inducing increased rates of cell division [15]. Medical nanorobots normally will have chemically-inert nonleachable surfaces, but designers should ensure that all possible nanorobot effluents are noncarcinogenic. (Potential nanorobot effluents may be prescreened during design using existing computational toxicology techniques [3].)

(2) Nonspecific Carcinogenesis. Neoplasms can arise in response to chronic irritation, leading to chronic inflammation and granulomatous reaction to implants [2]. Chemicals, foreign bodies, infection and mechanical trauma [16] can lead to this type of neoplastic transformation which is characterized by replication infidelity – i.e., a cell that produces a daughter cell not identical to its parent, as in, for example, the formation of hyperplastic expansive scars known as nonmalignant keloids [4]. These benign lesions can occasionally, and apparently spontaneously, transform into malignant neoplasms such as fibrous histiocytomas.

(3) Ex Cyto Foreign Body Carcinogenesis. In the 1950s it was discovered that many agents not previously thought to be carcinogenic produced dramatic neoplasm incidence rates in rodents when implanted in solid form rather than injected or fed in soluble or dispersed form, an effect called foreign body (FB) carcinogenesis [17-20], solid state carcinogenesis, or the Oppenheimer effect. The induction of neoplasms increases with the size of the implant and with decreasing inflammatory response (e.g., well-tolerated materials are, in the long run, better FB carcinogens). The risk of transformation is reduced on surfaces with porosity of average diameter above 220 nm, and materials with distributed porosity of cellular dimensions are less carcinogenic in rodents than smooth nonporous material [4, 19]. Nonperforated polymer films induce subcutaneous sarcomas in mice and rats, but implanted foreign bodies with other shapes (e.g., perforated or minced films, or filters with 450-nm pores [19]) or with roughened surfaces are weakly or non-carcinogenic except when total foreign-body surface area exceeds ~1 mm2 [20]. In vitro experiments by Boone et al. [18] and in vivo experiments by Brand [17] studied the effects of attachment of mouse fibroblasts to polycarbonate plates. Cells implanted after an in vitro exposure produced transplantable, undifferentiated sarcomas, leading these authors to conclude that the smooth surface of the plates acted as an FB carcinogen for at least initiation of tumorigenesis, independent of chemical composition. Brand [17] cited six possible mechanistic origins of FB carcinogenesis and concluded that: (1) disturbance of cellular growth regulation was most likely, based on the heritability of neoplastic behavior in the growing cell population, and (2) interruption of cellular contact or communication might also play a role in neoplasm expression and maturation (rather than neoplasm induction). It is now well-established that smooth-surfaced foreign bodies, regardless of their chemical composition, will produce sarcomas when transplanted subcutaneously into rodents [18], and foreign-body sarcomatous growth in mice appears resistant to substances which normally inhibit neoplastic growth.

Is there any evidence that humans are also susceptible to ex cyto FB carcinogenesis? There is no evidence that a single incident of mechanical trauma can cause cancer [21], but evidently there are 28 known cases of tumors in humans associated with partial or total joint replacements, which occurred either fairly soon after implantation or a very long time (10-15 years) after implantation, the latter primarily as malignant fibrous histiocytomas [4]. However, all of these tumors were associated with stainless steel or cobalt-based alloy devices, perhaps due to elevated tissue concentrations of metals near the implant [4] (e.g., metal-on-metal devices can produce a 10- to 15-fold rise in circulating serum chromium). There are a few additional reports of possible remote-site tumors [4], but other studies find such implant-related tumorigenicity to be very weak or nonexistent. Some investigators have therefore concluded that there is little clinical evidence for ex cyto FB carcinogenesis in humans, and that the Oppenheimer effect may be a consequence of the relatively primitive immune system of rodents in comparison to that of humans [4]. But Black [4] urges caution because, in rare cases, sarcomas appear to have arisen on unabsorbable foreign bodies in man [20] – a category of foreign bodies which would definitely include diamondoid medical nanorobots and nanoorgans. Polarizable foreign particles have also been associated with cutaneous granulomas in three cases of systemic sarcoidosis [22].

(4) In Cyto Foreign Body Carcinogenesis. Although FB carcinogenesis produced by materials external to cells appears to be rare in humans, solid materials in a form that can penetrate cells can be carcinogenic, a phenomenon originally known as the Stanton hypothesis [23]. The best-known example is chrysotile asbestos, first recognized as a human carcinogen only because it produced a relatively rare lung tumor. Subsequent studies of asbestos and related fibers in animal models revealed that mesothelioma could be induced by fibers <~0.25-1.5 microns in diameter and >~4-8 microns in length, regardless of fiber composition [23, 24]. Quantitatively, Stanton [23] found that ~105 fibers of carcinogenic dimensions, embedded in the human body, yielded a ~10% probability of developing a tumor within 1 year; ~2 x 107 fibers raised the probability to 50%; and 109 fibers, 90%. In vitro fiber cytotoxicity correlates well with fiber dimensions [24], particularly the aspect ratio, with fiber durability, and not with fiber bulk composition but rather with the molecular nature of active surface properties which can also play a role in carcinogenic potency [2]. Stiff slender fibers such as mineral whiskers can penetrate cells and may produce mechanical or oxidoreduction damage to the nucleus and to chromosomes [25] regardless of the material of which they are comprised. The likely mechanism is oxyradical activity because antioxidant enzymes appear to protect cells against genotoxic damage induced by chrysotile fibers. This risk factor must be borne in mind when designing medical nanorobots (including all of their possible operational and failure-mode physical configurations) and any potentially detachable subsystems which may be of similar stiffness and size as the cytotoxic fibers.

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